The present invention relates to a laser welding method for welding materials with a laser beam and to an apparatus for practicing this method. More particularly, the invention relates to a laser welding method in which a plurality of materials to be welded are stacked one on another and welded together and to an apparatus for practicing the method.
A laser welding apparatus of this type is disclosed in Japanese Laid-Open Patent Application No. 47590/1983. In that apparatus, upper and lower rollers are arranged in such a manner that materials to be welded which have been stacked together are held thereby, and the upper roller presses the materials to be welded while being rotated. The welding operation is carried out by applying a laser beam to the materials to be welded, which are moved horizontally while being held between the upper and lower rollers.
In the conventional welding machine constructed as described above, in the case where materials to be welded are coated members formed by plating or coating base plates such as steel plates with zinc or tin, or more specifically, when the materials to be welded are zinc-plated steel plates as shown in FIG. 1, when the zinc-plated steel plates 1 and 2 are rendered molten by a laser beam 3, the zinc deposits 4 through 7 becomes zinc vapor 8 instantaneously because the melting point of the zinc deposits 4 through 7 is much lower than that of the steel base. In this case, as the zinc-plated steel plates 1 and 2 are forced into close contact with each other, the zinc vapor 8 is forced to jet into a hole 9 formed by molten metal 10, thus blowing the molten metal. As a result, the hole 9 become irregular in configuration. Thus, during the welding operation, the molten metal cannot completely fill the hole 9, and therefore many voids 12 are liable to be formed in the solidified metal 11. This is a disadvantage of the conventional welding machine.